Exemplary embodiments of this disclosure relate to a semiconductor memory device and a method of operating the same.
A semiconductor memory device includes a cell array having a number of memory cells for storing data. The cell array includes a plurality of cell strings. Each of the cell strings includes two selection transistors and memory cells coupled in series between them. Furthermore, word lines are coupled to the gates of the respective memory cells of each of the cell strings.
When the memory cells are programmed, a program voltage is supplied to a selected word line and a pass voltage is supplied to the remaining unselected word lines.
At this time, there may be a memory cell that should not be programmed, from among memory cells coupled to the same word line. For this reason, a self-boosting method is employed in order to boost the channel voltage of a cell string coupled to a memory cell which should not be programmed.
In the self-boosting method employed in a program operation, 0 V is supplied to a bit line coupled to a memory cell to be programmed from among bit lines coupled to respective cell strings, and a power source voltage is supplied to a bit line coupled to a memory cell to be program-inhibited from among the bit lines. Furthermore, select transistors coupled between the bit lines and the cell strings are turned on, and the remaining selection transistors are turned off.
Assuming that the memory cells are in an erase state prior to the program operation and the power source voltage has been supplied to a bit line, the channel of the cell string coupled to the bit line is precharged to the difference between the power source voltage and the threshold voltage of the selection transistor.
Furthermore, when the program voltage is supplied to the selected word line and the pass voltage is supplied to the unselected word lines, the channel voltage of the precharge state rises, and thus the selection transistors are turned off according to the difference between the voltage of the bit line and the channel voltage. As a result, the channel is floated, and the channel voltage is further boosted by the program voltage. The channel of the cell string coupled to the memory cell to be programmed and to the bit line supplied with 0 V maintains 0 V. As a result, the memory cell to be programmed is programmed depending on the difference between the channel voltage of 0 V and the program voltage supplied to the selected word line.
Furthermore, in case of the memory cell to be program-inhibited, the channel voltage of the relevant cell string is highly boosted, and thus the difference between the channel voltage and the program voltage supplied to the selected word line is significantly reduced so that the memory cell is program-inhibited.
In the self-boosting method, one of important factors is the pass voltage supplied to the unselected word lines. A program inhibition effect may be increased because the significantly higher channel voltage is generated by the self-boosting as the higher pass voltage is supplied.
If a too high channel voltage is generated, however, a pass disturbance phenomenon, in which the memory cells coupled to the unselected word lines are undesirably programmed, may be generated. For this reason, a proper pass voltage must be supplied in order to maximize a program effect and reduce the pass disturbance phenomenon.
Furthermore, even when data is read out from a memory cell programmed as described above, a pass voltage is supplied to unselected word lines. Accordingly, a pass disturbance phenomenon may be generated because of the pass voltage.